(a) Field of the invention:
This invention relates to an image pickup system for endoscopes which, in an optical instrument provided with an image guide fiber bundle such as a fiberscope, picks up an image formed on an end face of the image guide fiber bundle.
(b) Description of the prior art:
Of recent years, it has become popular that a TV camera is attached to an eyepiece section of a fiberscope or the like to observe the body cavity with a TV monitor.
An end face of an image guide fiber bundle used in the fiberscope has a regular brightness pattern (screen dot structure) depending on a core array of fibers. With respect to the TV camera, on the other hand, in the case where a pickup tube is used and a color striped filter is provided before its light receiving surface and in the case where a solid-state image pickup device is used and a color mosaic filter is provided before it, color elements of each filter exhibit an ordered array, and even in the case where the mosaic filter is not disposed, pixel elements of the solid-state image pickup device assume an ordered array. Thus, a problem has been encountered that the preceding of the filter bundle regular structure relative to the irregular structure of the image guide fiber bundle give rise to interference of light, resulting in generation of moire in a TV picture image. Here, the color striped filter or the color mosaic filter represents a color encoding filter configured by arraying color elements, in a stripe pattern or a mosaic pattern, composed of minute filters with additive primary colors or subtractive primary colors.
For a technology to eliminate the moire of this type, it is known that an optical low-pass filter is provided between the exit end face of the image guide fiber bundle and the solid-state image pickup device. For example, Japanese Patent Preliminary Publication No. Sho 55-143125 sets forth the use of a phase filter as the optical low-pass filter. Further, Japanese Patent Preliminary Publication No. Sho 59-193416 makes use of a birefringent filter combined with a quartz plate as the optical low-pass filter.
However, various types of fiberscopes have been developed of late, which range, for instance, from an extremely fine one (approximately 0.5 mm in diameter) for viewing the inside of blood vessels to a considerably thick one for large intestines, thicker in endoscopes for industry. In the case where these parts are observed with the same TV camera, it is required that images are made similar in size to each other in the use of any fiberscope, so that magnification in imaging the end face of the image guide fiber bundle on the solid-state image pickup device varies widely for each fiberscope and consequently cores in each image (namely, on an image pickup surface) will largely be different in thickness from each other. This will be explained in relation to frequency domain as follows:
The solid-state image pickup device is adapted to sample an object image in a spatial frequency corresponding to the repetition period of the regular structure mentioned above. As known from communication theory, if the frequency spectrum zone of a signal to be sampled reaches the range of a high-frequency wave exceeding a Nyquist rate, what is called aliasing will be generated to bring about moire.
In this case, the signal to be sampled represents an image formed on the exit end face of the image guide fiber bundle and, if brightness variation caused by repetition of cores is taken as a sine wave, its spatial frequency spectrum will range to the repetition frequency. Since the brightness variation is not represented by the sine wave in fact, a harmonic wave component also exists and a fundamental wave component is largest as an extent of the spectrum.
Accordingly, an interpretation may be given such that the band width of the spatial frequency spectrum of the image is substantially determined by the repetition period of cores in the image. This means that the aspect of generation of moire is severely affected by thicknesses of cores in the image formed on the exit end face of the image guide fiber bundle which is transmitted onto the image pickup device.
Since spectrum components of the spatial frequency depending on the core period are very large, the generation of moire is mainly attributable to such components.
The relationship between the Nyquist rate of the solid-state image pickup device and the spatial frequency of cores in the image formed on the exit end face of the image guide fiber bundle is assumed as shown in FIGS. 1 to 3.
Each of these figures represents a two-dimensional spatial frequency plane, in which reference symbol f.sub.H is the axis indicative of the frequency in a horizontal scanning direction and f.sub.V the frequency in a vertical scanning direction. The Nyquist rates in the horizontal and vertical directions of the solid-state image pickup device are taken as f.sub.HN and f.sub.VN, respectively. Further, when, among the fibers used in the fiberscope, the smallest diameter is represented by .phi. and the largest one by .phi.', and when the minimum value of magnification in the case where the exit end face of the image guide fiber bundle is imaged on the solid-state image pickup device is taken as .beta. and the maximum one as .beta.', the values of the thicknesses of cores in the image formed on the solid-state image pickup device lie within the range of .phi..beta..about..phi.'.beta.'. Thus, the spatial frequencies of cores are distributed over the range of 1/.phi.'.beta.'.about.1/.phi..beta..
Since FIG. 3 shows the example that the core frequency does not exceed the Nyquist rate (in other wards, thicker cores are imaged) and no moire appears, it is only necessary to discuss the examples of FIGS. 1 and 2.
In the example of FIG. 1, the core spatial frequency is higher than the Nyquist rate of the solid-state image pickup device and as such, if the optical low-pass filter is provided to eliminate the spatial frequency components of cores, the generation of moire is extremely diminished (to a practically negligible extent). The image is blurred or a multiple image whose elements are slightly shifted is formed by the optical low-pass filter in outward appearance and thereby the spaces among the cores are filled so that the screen dot structure of the cores is not viewed. Also, such a manner has little effect on the resolution of the TV picture image to be photographed since the spatial frequency components are eliminated outside the range of the Nyquist frequency of the solid-state image pickup device.
The case of FIG. 2, in which the Nyquist rate is included in the distribution band of the core spatial frequency, has problems. In this example, the core frequency f.sub.IG is larger than the Nyquist rate f.sub.VN with respect to its vertical direction, while on the other hand, in its horizontal direction, the core frequency f.sub.IG is smaller than the Nyquist rate f.sub.HN, that is, the frequency domain such that the relationship of f.sub.VN &lt;F.sub.IG &lt;f.sub.HN is established exists. Although the core frequency in the vertical direction, like the example of FIG. 1, is favorable because the optical low-pass filter is provided such that a response is reduced to zero in the distribution band of the core spatial frequency, the response is reduced, in the horizontal direction, on the inside considerably farther (at a lower frequency) than the Nyquist rate, with the result that the reduction of resolution is inevitably brought about.